SumOfAllDigitsValidator

// A number is considered valid if and only if the sum of all its digits is divisible by 7
// and the product of all its digits is divisible by 5.

public class SumOfDigitsValidator {

    public static void main(String []args) {

        int count = 0;

        for (int i=0; i <= 9999; i++) {
            if (isValidNumber(sumOfAllDigits(i), productOfAllDigits(i))) {
                count++;
            }
        }

        System.out.printf("\nThere are exactly %,d valid numbers between 0 and 9999.\n",count);
    }

    public static int sumOfAllDigits(int entry) {
        return sumOfAllDigitsHelper(entry,0);
    }

    private static int sumOfAllDigitsHelper(int entry, int sum) {
        if (entry == 0) {
            return sum;
        }
        sum += entry % 10;
        return sumOfAllDigitsHelper(entry/10, sum);

    }

    public static int productOfAllDigits(int entry) {
        return productOfAllDigitsHelper(entry,1);
    }

    private static int productOfAllDigitsHelper(int entry, int product) {
        if (entry == 0) {
            return product;
        }
        product *= entry % 10;
        return productOfAllDigitsHelper(entry/10, product);
    }

    private static boolean isValidNumber(int sum, int product) {
        return sum % 7 == 0 && product % 5 == 0;
    }
}